Method of producing cold-rolled steel sheet as well as cold-rolled steel sheet and members for automobile

ABSTRACT

In a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability but also the corrosion resistance after coating under severe corrosion environment such as hot salt water immersion test or composite cycle corrosion test, a continuously annealed steel sheet after cold rolling is pickled with a mixture of nitric acid and hydrochloric acid having a nitric acid concentration of more than 100 g/L but not more than 200 g/L and a ratio R (HCl/HNO 3 ) of hydrochloric acid concentration to nitric acid concentration of 0.01-0.25 to remove Si-containing oxide formed on the steel sheet surface by continuous annealing, and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85% and preferably a maximum thickness of the iron-based oxide existing on the surface of the steel sheet is not more than 200 nm.

TECHNICAL FIELD

This invention relates to a method of producing a cold-rolled steel sheet as well as a cold-rolled steel sheet and a member for automobile, and more particularly to a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability but also the corrosion resistance after coating as evaluated by a hot salt water immersion test or a composite cycle corrosion test, a cold-rolled steel sheet produced by this method as well as a member for automobile using the cold-rolled steel sheet. Moreover, the cold-rolled steel sheet according to the invention can be preferably used in a high-strength cold-rolled steel sheet containing Si and having a tensile strength TS of not less than 590 MPa.

BACKGROUND ART

Recently, it is strongly demanded to improve fuel consumption of an automobile from a viewpoint of the protection of global environment. Also, it is strongly demanded to improve the safety of the automobile from a viewpoint of ensuring the safe of crew members at the time of impact. In order to meet these demands, it is required to simultaneously attain weight reduction and high-strengthening of a vehicle body in the automobile, while the thinning associated with the high strengthening is positively proceeding in cold-rolled steel sheets as a starting material in the member for automobile. However, many members for automobile are manufactured by forming the steel sheet, so that these steel sheets are required to have an excellent formability in addition to the high strength.

There are various methods for enhancing the strength of the cold-rolled steel sheet. As a method increasing the strength without largely damaging the formability is mentioned a solid-solution strengthening method through addition of Si. However, when a greater amount of Si, particularly not less than 0.5 mass % of Si is added to a cold-rolled steel sheet, it is known that Si-containing oxides such as SiO₂, Si—Mn based composite oxide and the like are formed on the surface of the steel sheet during slab heating or during annealing after hot rolling or after cold rolling. Since the Si-containing oxide considerably deteriorates the phosphate treatability, the high-strength cold-rolled steel sheets containing a great amount of Si have problems that the phosphate treatability is poor but the coating peeling is easily caused to deteriorate the corrosion resistance after the coating as compared with the commonly used steel sheets when the steels sheet after electrodeposition coating is subjected to severer corrosion environment as in a hot salt water immersion test or a composite cycle corrosion test repeating cycle of wetting-drying.

As a countermeasure for these problems, for example, Patent Document 1 proposes a high-strength cold-rolled steel sheet obtained by heating a slab at a temperature of higher than 1200° C. in hot rolling, descaling under high pressure, polishing the surface of the hot-rolled steel sheet with a nylon brush containing abrasion grains prior to pickling and then immersing in a bath of 9% hydrochloric acid twice to perform pickling to lower the Si concentration on the surface of the steel sheet. Also, Patent Document 2 proposes a high-strength cold-rolled steel sheet wherein the corrosion resistance is improved by rendering line width of Si-containing linear oxide observed in 1-10 μm from the surface of the steel sheet into not more than 300 nm.

However, in the high-strength cold-rolled steel sheet disclosed in Patent Document 1, even if the Si concentration on the surface of the steel sheet is reduced before the cold rolling, the Si-containing oxide is formed on the surface of the steel sheet by annealing after cold rolling, so that the improvement of the corrosion resistance after coating is not desired. Also, in the high-strength cold-rolled steel sheet disclosed in Patent Document 2, there is no problem in the corrosion resistance under corrosion environment as in a salt spray test defined according to JIS Z2371, but sufficient corrosion resistance after coating is not obtained under severer corrosion environment as in a hot salt water immersion test or a composite cycle corrosion test. That is, the high-strength cold-rolled steel sheet having an excellent corrosion resistance after coating can not be obtained only by reducing the Si concentration on the surface of the steel sheet after hot rolling or by reducing the Si-containing linear oxide.

As a technique for solving the above problems, Patent Document 3 discloses a technique wherein the Si-containing oxide enriched on the surface of the steel sheet by annealing step or the like is removed by pickling and further an S-based compound is applied to the surface to enhance the reactivity with a phosphate treating solution to thereby improve the phosphate treatability. Also, Patent Document 4 discloses a technique wherein a P-based compound is applied instead of the S-based compound of the above technique.

PRIOR ART ARTICLES Patent Document

[Patent Document 1] JP-A-2004-204350

[Patent Document 2] JP-A-2004-244698

[Patent Document 3] JP-A-2007-217743

[Patent Document 4] JP-A-2007-246951

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In recent years, for the purpose of reducing industrial wastes (suppression of sludge formation) and cutting down running cost, it is proceeded to lower the temperature of the phosphate treating solution, and hence the reactivity of the phosphate treating solution to the steel sheet is largely lowered as compared with the conventional phosphate treating conditions. The lowering of the temperature of the treating solution does not come into problem when the surface adjusting technique prior to the phosphate treatment is improved in the common steel sheet having a less addition amount of alloy usually used. However, in the high-strength cold-rolled steel sheet added with a great amount of Si, the reactivity with the phosphate treating solution is considerably deteriorated by the influence of the Si-containing oxide formed on the surface of the steel sheet at an annealing step, so that it is required to enhance the reactivity from the steel sheet side in some way. On the other hand, the techniques disclosed in Patent Documents 3 and 4 are effective to the conventional common steel sheets, but can not expect the sufficient improving effect capable of lowering the temperature of the phosphate treating solution for the high-strength cold-rolled steel sheets containing a great amount of Si.

The invention is made in view of considering the above problems inherent to the cold-rolled steel sheet containing a great amount of Si and is to provide a method of producing a cold-rolled steel sheet being excellent in not only the phosphate treatability even when using a phosphate treating solution at a lower temperature but also in the corrosion resistance after coating as evaluated by a hot salt water immersion test or a composite cycle corrosion test, a cold-rolled steel sheet produced by this method as well as a member for automobile using the cold-rolled steel sheet.

Means for Solving Problems

The inventors have made detailed analysis on surface properties of steel sheets after annealing in order to solve the above problems and various studies on a method of enhancing the reactivity between the surface of the steel sheet and the phosphate treating solution. As a result, it has been found that it is very important to subject the continuously annealed steel sheet surface to strong pickling after the cold rolling to thereby remove Si-containing oxide layer formed on the surface of the steel sheet during the annealing but also reduce a ratio of covering the surface of the steel sheet with an iron-based oxide formed on the steel sheet surface by the strong pickling, and consequently the invention has been accomplished.

That is, the invention proposes a method of producing a cold-rolled steel sheet, characterized in that a continuously annealed steel sheet after cold rolling is pickled with a mixture of nitric acid and hydrochloric acid having a nitric acid concentration of more than 100 g/L but not more than 200 g/L and a ratio R (HCl/HNO₃) of hydrochloric acid concentration to nitric acid concentration of 0.01-0.25.

The mixture of nitric acid and hydrochloric acid in the production method of the invention is characterized to have a nitric acid concentration of more than 110 g/L but not more than 140 g/L and a ratio R (HCl/HNO₃) of hydrochloric acid concentration to nitric acid concentration of 0.03-0.25.

Also, the production method of the invention is characterized in that the pickling is carried out at a temperature of a pickling solution of 20-70° C. for 3-30 seconds.

The steel sheet in the production method of the invention is characterized by a chemical composition comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities.

Also, the cold-rolled steel sheet in the production method of the invention is characterized by comprising, in addition to the above chemical composition, one or more selected from the group consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.

Furthermore, the cold-rolled steel sheet in the production method of the invention is characterized by comprising, in addition to the above chemical composition, Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.

Moreover, the invention is a cold-rolled steel sheet produced by any aforementioned method, which is a cold-rolled steel sheet characterized in that a Si-containing oxide layer is removed from the surface of the steel sheet by pickling after continuous annealing and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85%.

The cold-rolled steel sheet according to the invention is characterized in that a maximum thickness of the iron-based oxide existing on the surface of the steel sheet is not more than 200 nm.

Further, the invention is a member for automobile characterized by using any aforementioned cold-rolled steel sheet.

Effect of the Invention

According to the invention, there can be provided a cold-rolled steel sheet which is excellent in the phosphate treatability even when Si is contained as large as 0.5-3.0 mass% and when using a phosphate treating solution at a lower temperature but also is excellent in the corrosion resistance after coating under severer corrosion environment as in a hot salt water immersion test or a composite cycle corrosion test. According to the invention, therefore, it is possible to largely improve the phosphate treatability and corrosion resistance after coating in the high-strength cold-rolled steel sheets containing a greater amount of Si and having a tensile strength TS of not less than 590 MPa, so that it can be preferably used in strong members and the like in a vehicle body of an automobile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows reflection electron microphotographs of steel sheet surfaces of standard cold-rolled steel sheet sample Nos. a and b for determining a surface covering ratio with an iron-based oxide.

FIG. 2 shows a histogram of pixel number to gray value in the reflection electron microphotographs of the standard cold-rolled steel sheet sample Nos. a and b.

FIG. 3 is a photograph of a section of a coating on a surface of a steel sheet after pickling observed by means of a transmission electron microscope.

FIG. 4 is a graph showing energy dispersion type X-ray (EDX) analytical results of an iron-based oxide observed in FIG. 3.

FIG. 5 is a graph of depth distribution of O, Si, Mn and Fe on a surface of a test specimen in Comparative Example (No. 1) and Invention Example (No. 18) of Example 1 as measured by GDS.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

First, the basic technical idea of the invention will be described.

In an annealing step using a continuous annealing furnace for recrystallizing a cold-rolled steel sheet after cold rolling to impart desired structure, strength and workability, a non-oxidizing or reducing gas is usually used as an atmosphere gas, and also a dew point is strictly controlled. In the commonly general cold-rolled steel sheet having a less amount of an alloy added, therefore, the oxidation of the steel sheet surface is controlled. However, in the steel sheet containing not less than 0.5 mass % of Si or Mn, even if component or dew point of the atmosphere gas in the annealing is strictly controlled, it can not be avoided that Si, Mn and the like being easily oxidizable as compared with Fe are oxidized to form a Si-containing oxide such as Si oxide (SiO₂), Si—Mn based composite oxide or the like on the surface of the steel sheet. The construction of these oxides varies depending on components of the steel sheet, annealing atmosphere and the like, but both the oxides are typically and frequently existent in a mixture. Also, since the Si-containing oxide is formed not only the surface of the steel sheet but also in the interior of the steel matrix, it is known that the oxide obstructs the etching property on the surface of the steel sheet in the phosphate treatment (treatment with zinc phosphate) made as an underlaying treatment for electrodeposition coating and badly affects the formation of sound phosphate treated coating.

In recent years, the lowering of the temperature of the phosphate treating solution is proceeding for the purpose of reducing the sludge amount generated in the phosphate treatment and the running cost, and hence the phosphate treatment is carried out under a condition that the reactivity of the phosphate treating solution to the steel sheet is considerably low as compared with the conventional technique. The change of the phosphate treating condition is not particularly questioned by the improvement of the surface adjusting technique or the like in the conventionally used common steel sheets having a less addition amount of an alloy. In the steel sheet having a greater addition amount of alloying component, particularly a high-strength cold-rolled steel sheet attempted to increase the strength by adding a greater amount of Si, however, the influence of changing the phosphate treating condition as mentioned above is very large. In the cold-rolled steel sheet having a greater amount of Si, therefore, it is required that the surface of the steel sheet itself is activated in correspondence with the deterioration of the phosphate treating condition to enhance the reactivity with the phosphate treating solution.

The inventors have made various investigations on a method of improving the phosphate treatability for corresponding to the deterioration of the phosphate treating condition as mentioned above. As a result, it has been found out that it is effective to conduct strong pickling of the surface of the cold-rolled steel sheet after continuous annealing with nitric acid as a pickling solution to remove a Si-containing oxide layer formed on the surface of the steel sheet by continuous annealing and the like after cold rolling. The term “Si-containing oxide” used herein means SiO₂ or Si—Mn base composite oxide formed on the surface of the steel sheet or along crystal grain boundary inside the steel sheet in the slab heating or after hot rolling or in annealing after cold rolling. The thickness of the layer containing these Si-containing oxides varied depending upon components of the steel sheet or the annealing condition (temperature, time, atmosphere), but is usually about 1 μm from the surface of the steel sheet. Also, the term “removal of the Si-containing oxide layer” according to the invention means that the pickling is carried out to remove the Si-containing oxide layer to a level that peaks of Si, O do not appear when the surface of the steel sheet is analyzed in depth direction by means of GDS (glow discharge optical emission spectroscopy).

Moreover, the reason why nitric acid is used as the pickling solution is due to the fact that the Si—Mn based composite oxide among the Si-containing oxides is easily dissolved in the acid, but SiO₂ is hardly soluble, and in order to remove the latter, nitric acid as a strong-oxidizing acid must be used to remove the Si-containing oxide on the surface of the steel sheet together with the steel matrix.

According to the inventors' studies, however, it can be seen that the phosphate treatability is largely improved by conducting strong pickling with nitric acid after the continuous annealing to remove the Si-containing oxide layer existing on the surface of the steel sheet but the phosphate treatability may be deteriorated at moments. As the cause is further investigated, it is newly found that although the Si-containing oxide layer is removed by the strong pickling with nitric acid, Fe dissolved from the surface of the steel sheet by the pickling separately produces an iron-based oxide, which is settled and precipitated on the surface of the steel sheet so as to cover the steel sheet surface to thereby deteriorate the phosphate treatability.

Furthermore, it has been found that in order to suppress the oxidation of the steel sheet surface by the above pickling with nitric acid to mitigate the bad influence upon the phosphate treatability, it is important to suppress the formation of the iron-based oxide on the steel sheet surface to reduce the ratio of covering the steel sheet surface with the iron-based oxide to not more than 85% and that it is important as means for attaining the above to control the concentration of nitric acid to an adequate range to suppress the oxidation with nitric acid and to conduct the pickling with a mixture of nitric acid and hydrochloric acid at a given mixing ratio of hydrochloric acid having an effect of breaking an oxide film.

The inventors have also found that the phosphate treatability is more improved and the corrosion resistance is further improved when the covering ratio of the iron-based oxide generated on the surface of the steel sheet by pickling is not more than 85% and further the maximum thickness of the iron-based oxide is not more than 200 nm and that it is effective as means for attaining the above to control the concentration of hydrochloric acid having the effect of breaking an oxide film, which is used in a part of the pickling solution, to an adequate range for pickling.

Moreover, the iron-based oxide in the invention means an oxide composed mainly of iron wherein an atomic concentration ratio of iron is not less than 30% as an element other than oxygen constituting the oxide. The iron-based oxide is existent on the surface of the steel sheet at an uneven thickness, which is different from a natural oxide film existing uniformly and in layer at a thickness of few nm. The iron-based oxide generated on the surface of the cold-rolled steel sheet is confirmed to be amorphous from the observation by means of a transmission electron microscope (TEM) and analysis results of diffraction pattern (analytical diagram) through an electron diffractometry.

The invention is accomplished by conducting further examinations on the above new knowledge.

The reason why the chemical composition of the cold-rolled steel sheet according to the invention is limited to the above range will be described below.

Si: 0.5-3.0 mass %

Si is an element effective for attaining the increase of the strength of the steel because the effect of enhancing the strength of steel (solid-solution strengthening ability) is large without largely damaging the workability, but is also an element adversely exerting on the phosphate treatability and the corrosion resistance after coating. When Si is added as means for attaining a high strength, the addition of not less than 0.5 mass % is necessary. If the Si content is less than 0.5 mass %, the influence due to the deterioration of the phosphate treating conditions is less. On the other hand, when the Si content exceeds 3.0 mass %, the hot rolling property and cold rolling property are largely deteriorated, which is adversely influenced on the productivity and leads to the deterioration of ductility of the steel sheet itself. Therefore, Si is added within a range of 0.5-3.0 mass %. Preferably, it is a range of 0.8-2.5 mass %.

The cold-rolled steel sheet of the invention is an essential feature to include Si in the above range. The other components are acceptable as far as they are included within composition ranges in the common cold-rolled steel sheet, and are not particularly limited. However, the cold-rolled steel sheet of the invention is preferable to have the following component composition when it is applied to a high-strength cold-rolled steel sheet having a tensile strength of not less than 590 MPa for use in vehicle bodies for automobiles and so on.

C: 0.01-0.30 mass %

C is an element effective for enhancing the strength of steel and further is an element effective for producing residual austenite having an effect of TRIP (Transformation Induced Plasticity), bainite and martensite. When C content is not less than 0.01 mass %, the above effect is obtained, while when C content is not more than 0.30 mass %, the deterioration of the weldability is not caused. Therefore, C is added preferably within a range of 0.01-0.3 mass %, more preferably within a range of 0.10-0.20 mass %.

Mn: 1.0-7.5 mass %

Mn is an element having an action for solid-solution strengthening steel to increase the strength and enhance the hardenability and promoting the formation of residual austenite, bainite and martensite. Such effects are developed by the addition of not less than 1.0 mass %. On the other hand, when Mn content is not more than 7.5 mass %, the above effect is obtained without the increase of the cost. Therefore, Mn is added preferably within a range of 1.0-7.5 mass %, more preferably within a range of 2.0-5.0 mass %.

P: not more than 0.05 mass %

P is an element damaging no drawability though the solid-solution strengthening ability is large and is also an element effective for attaining a high strength, so that it is preferable to be included in an amount of not less than 0.005 mass %. However, P is an element damaging the spot weldability, but there is no problem when it is not more than 0.05 mass %. Therefore, P is preferably not more than 0.05 mass %, more preferably not more than 0.02 mass %.

S: not more than 0.01 mass %

S is an impurity element inevitably incorporated, and is a harmful element which is precipitated in steel as MnS to deteriorate the stretch-flanging property. In order to prevent the deterioration of the stretch-flanging property, S is preferably not more than 0.01 mass %, more preferably not more than 0.005 mass %, further preferably not more than 0.003 mass %.

Al: not more than 0.06 mass %

Al is an element added as a deoxidizer at steel-making step, and is also an element effective for separating non-metallic inclusion, which deteriorates the stretch-flanging property, as a slug, so that it is preferable to be included in an amount of not less than 0.01 mass %. When Al content is not more than 0.06 mass %, the above effect is obtained without the increase of cost for material. Therefore, Al is preferable to be not more than 0.06 mass %, More preferably, it is a range of 0.02-0.06 mass %.

In addition to the above components, the cold-rolled steel sheet of the invention may contain one or more selected from Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.

Nb, Ti and V are elements forming carbide and nitride to suppress ferrite growth at a heating stage in the annealing and finely divide the structure to improve the formability, particularly stretch-flanging property, and also Mo, Cr and B are elements improving the hardenability of steel and promoting the formation of bainite and martensite, so that they can be added within the above ranges. Also, N is an element forming nitrides with Nb, Ti and V or solid-soluting in steel to contribute to the increase of the strength of steel, so that when it is not more than 0.008 mass %, a greater amount of the nitride is not formed, and hence the breakage due to the formation of voids in the press forming can be suppressed to obtain the above effect.

In addition to the above components, the cold-rolled steel sheet of the invention may contain one or more selected from Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.

Ni and Cu promote the formation of the low-temperature transformation phase to develop the effect of increasing the strength of steel, so that they can be added within the above ranges. Also, Ca and REM are elements controlling the form of the sulfide base inclusion to improve the stretch-flanging property of the steel sheet, so that they can be added within the above ranges.

In the cold-rolled steel sheet of the invention, the remainder other than the above components is Fe and inevitable impurities. However, other components may be optionally added within a scope of not damaging the action and effect of the invention.

The surface properties of the cold-rolled steel sheet of the invention will be described below.

As mentioned above, the cold-rolled steel sheet of the invention is necessary to have a steel sheet surface obtained after the removal of Si-containing oxide layer such as SiO₂ or Si—Mn based composite oxide layer formed on the surface layer of the steel sheet during annealing. For this end, it is necessary to conduct strong pickling with a pickling solution of mixed nitric acid and hydrochloric acid to dissolve and remove the Si-containing oxide formed on the surface of the steel sheet and in the grain boundary portion in the vicinity of the surface together with the steel matrix.

Furthermore, in the cold-rolled steel sheet of the invention, it is necessary to reduce the ratio of covering the surface of the steel sheet with iron-based oxide generated on the steel sheet surface by the strong pickling with nitric acid to not more than 85% as an area ratio in addition to the removal of the Si-containing oxide layer. When the surface covering ratio of the iron-based oxide exceeds 85%, the dissolving reaction of iron in the phosphate treatment is inhibited to suppress the crystal growth of phosphate such as zinc phosphate or the like. It is preferably not more than 80%.

In the invention, the surface covering ratio of the iron-based oxide is determined as follows:

The surface of the steel sheet after the pickling is observed at about 5 fields with a ultra-low acceleration voltage scanning type electron microscope (ULV-SEM) capable of detecting information of an extremely surface layer under conditions of acceleration voltage: 2 kV, operating distance: 3.0 mm and magnification: about 1000 times and spectroscopy is conducted with an energy dispersion type X-ray spectrometer (EDX) to obtain a reflection electron image. The reflection electron image is binarized with an image analysis software, e.g. Image J to measure an area ratio of a black portion. The measured results on the fields can be averaged to obtain a surface covering ratio of the iron-based oxide. Moreover, as the ultra-low acceleration voltage scanning type electron microscope

(ULV-SEM) may be mentioned, for example, ULTRA 55 made by SEISS, and as the energy dispersion type X-ray spectrometer (EDX) may be mentioned, for example, NSS 312E made by Thermo Fisher.

Here, threshold value in the binarization will be described.

A steel slab of Steel symbol G shown in Table 3 of the following example is subjected to hot rolling, cold rolling and continuous annealing under conditions of No. 7 in Table 4 of the following example to obtain a cold-rolled steel sheet of 1.8 mm in thickness, and then the cod-rolled steel sheet after the continuous annealing is subjected to pickling and repickling under conditions shown in Table 1, washed with water, dried and subjected to 0.7% temper rolling to obtain two cold-rolled steel sheets of Nos. a and b having different iron-based oxide amounts on their steel sheet surfaces. Then, the cold-rolled steel sheet of No. a is a standard sample having a large amount of iron-based oxide and the cold-rolled steel sheet of No. b is a standard sample having a small amount of iron-based oxide, and each of these steel sheets is observed with the scanning type electron microscope under the aforementioned conditions to obtain a reflection electron image. FIG. 1 shows photographs of reflection electron images of steel sheets Nos. a and b, and FIG. 2 shows a histogram of pixel number to a gray value in the photographs of the reflection electron images of the steel sheets Nos. a and b. In the invention, a gray value (Y point) corresponding to an intersecting point (X point) of the histograms of Nos. a and b shown in FIG. 2 is defined as a threshold value. Incidentally, when the surface covering ratio of the iron-based oxide in the steel sheets Nos. a and b is determined with the above threshold value, it is 85.3% in the steel sheet No. a and 25.8% in the steel sheet No. b.

TABLE 1 Pickling conditions Treating Repickling conditions Surface covering ratio Steel Acid concentration Temperature time Acid concentration Temperature Treating time of iron-based oxide sheet (g/l) (° C.) (Seconds) (g/l) (° C.) (Seconds) (%) a Nitric acid: 250 + 40 10 — — — 85.3 Hydrochloric acid: 25 b Nitric acid: 150 + 40 10 Hydrochloric acid: 10 40 30 25.8 Hydrochloric acid: 15

In order to more improve the phosphate treatability and corrosion resistance in the cold-rolled steel sheet of the invention, it is preferable that the maximum thickness of the iron-based oxide is not more than 200 nm in addition that the covering ratio of the iron-based oxide produced on the steel sheet surface by pickling is not more than 85%. When the maximum thickness of the iron-based oxide is not more than 200 nm, the dissolving reaction of iron through the phosphate treatment is not inhibited locally and also the precipitation of crystal of phosphate such as zinc phosphate or the like is not inhibited locally. More preferably, it is not more than 180 nm.

The maximum thickness of the iron-based oxide is measured as follows.

First, 10 extraction replicas are prepared from the surface of the steel sheet after the pickling by a focused ion beam (FIB) work for observing a section of about 8 μm relative to the widthwise direction of the steel sheet. Then, the section of 8 μm in the each replica is continuously shot by means of a transmission electron microscope (TEM) provided with an energy dispersion type X-ray spectrometer

(EDX) capable of checking local information of the section at an acceleration voltage of 200 kV and a magnification of 100000 times. As an example, FIG. 3 is a photograph showing a section of a covering layer existing on the surface of the steel sheet and generated by pickling as observed by TEM, and FIG. 4 shows analytical results of the covering layer by EDX. As seen from FIG. 4, the covering layer is an iron-based oxide composed mainly of iron. Therefore, the interval between a line A showing a surface of the steel sheet and a line B showing a thickest portion of an oxide layer shown by the photograph of the section in FIG. 3 is measured with respect to the 10 replicas, and a maximum thickness among them is a maximum thickness of the iron-based oxide. Moreover, the size and numbers of the replicas, measuring conditions by TEM and the like as mentioned above are merely exemplified, and may be properly modified as a matter of course.

The production method of the cold-rolled steel sheet according to the invention will be described below.

The production method of the cold-rolled steel sheet of the invention is necessary to be a method wherein a steel material (slab) having Si: 0.5-3.0 mass % is heated, hot rolled, cold rolled, continuously annealed and then pickled with a pickling solution of mixed nitric acid and hydrochloric acid, whereby a Si-containing oxide layer is removed from a surface layer portion of the steel sheet and a surface covering ratio of an iron-based oxide generated on the surface of the steel sheet by pickling can be made to not more than 85%. Further, it is preferable to be a method wherein a maximum thickness of the iron-based oxide can be made to not more than 200 nm. Therefore, the procedure ranging from the steel-making step to the continuous annealing step after the cold rolling can be carried out according to the usual manner, but the pickling after the continuous annealing is preferable to be conducted under the following conditions.

Pickling conditions after continuous annealing

On the surface layer of the steel sheet after the continuous annealing is produced a greater amount of the Si-containing oxide such as SiO₂, Si—Mn based composite oxide or the like, so that the phosphate treatability and the corrosion resistance after coating are considerably deteriorated. In the production method of the invention, therefore, it is necessary that the cold-rolled steel sheet after the annealing is strongly pickled with a pickling solution of mixed nitric acid and hydrochloric acid, whereby the Si-containing oxide layer on the surface of the steel sheet is removed with the steel matrix but also the formation of the iron-based oxide settled and precipitated on the surface of the steel sheet by pickling is suppressed.

As previously mentioned, Si—Mn based composite oxide among the Si-containing oxides is easily dissolved in an acid, but SiO₂ is insoluble in an acid. Therefore, in order to remove the Si-containing oxide including SiO₂ by pickling, the steel matrix is required to be removed with nitric acid as a strong acid. In order to conduct the strong pickling for removing the oxide layer including the steel matrix, it is necessary that the concentration of nitric acid is more than 100 g/L. However, since nitric acid is a strongly oxidizing acid, Fe eluted is oxidized to form an iron-based oxide, which is precipitated on the surface of the steel sheet and adversely affects the phosphate treatability and the corrosion resistance after coating. In order to suppress this adverse effect, therefore, it is necessary to control the concentration of nitric acid to not more than 200 g/L. Thus, the concentration of nitric acid is more than 100 g/L but not more than 200 g/L. Preferably, it is a range of 110-150 g/L.

However, when the concentration of nitric acid is merely limited to the above range, it is difficult to stably control the surface covering ratio of the iron-based oxide generated on the surface of the steel sheet by pickling with nitric acid to not more than 85%. In the invention, therefore, in order to more surely suppress the formation of the iron-based oxide on the surface of the steel sheet by strong pickling with nitric acid, the pickling is carried out with such a mixed acid that the concentration of nitric acid is limited to the above range but also a ratio R (HCl/HNO₃) of a concentration of a chloride ion having an effect of breaking an oxide film, i.e. hydrochloric acid to the concentration of nitric acid is a range of 0.01-0.25. When the ratio R is less than 0.01, the effect of suppressing the formation of the iron-based oxide is small, while when it exceeds 0.25, the amount of the steel sheet dissolved is reduced and hence the Si-containing oxide layer cannot be removed.

In order to more improve the phosphate treatability and the corrosion resistance, it is desirable to render the maximum thickness of the iron-based oxide generated on the surface of the steel sheet by pickling into not more than 200 nm. For this end, the mixed acid pickling solution of nitric acid and hydrochloric acid is preferable to have a concentration of nitric acid within a range of more than 110 g/L but not more than 140 g/L and a ratio R (HCl/HNO₃) of hydrochloric acid concentration to nitric acid concentration within a range of 0.03-0.25. When the concentrations are satisfied within the above ranges, it is possible to stably make the thickness of the iron-based oxide to not more than 200 nm, and hence the phosphate treatability and the corrosion resistance after coating are not deteriorated.

Moreover, it is preferable that the pickling with the mixed pickling solution of nitric acid and hydrochloric acid is carried out at a temperature of the pickling solution of 20-70° C. for a pickling time of 3-30 seconds. When the temperature of the pickling solution is not lower than 20° C. and the pickling time is not less than 3 seconds, the Si-containing oxide layer formed in the surface layer of the steel sheet in the annealing can be removed sufficiently, and the phosphate treatability and the corrosion resistance after coating are never deteriorated. While, when the temperature of the pickling solution is not higher than 70° C. and the time is not more than 30 seconds, there is no phenomenon due to excessive pickling that the surface of the steel sheet becomes coarse and the phosphate coating becomes non-uniform and the surface covering ratio of the iron-based oxide becomes high, and the phosphate treatability and the corrosion resistance after coating are never deteriorated.

The cold-rolled steel sheet, wherein the covering ratio of the steel sheet surface with the iron-based oxide is made to not more than 85% by pickling after the continuous annealing as mentioned above, or alternately the cold-rolled steel sheet, wherein the maximum thickness of the iron-based oxide is made to not more than 200 nm, is subsequently subjected to usual treating steps such as temper rolling and the like to provide products.

Example 1

A steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6 mass %, P: 0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the remainder being Fe and inevitable impurities is prepared according to common refining process such as melting in a converter, degassing treatment and the like and continuously cast into a steel material (slab). The slab is reheated to a temperature of 1150-1170° C., hot rolled at a terminating temperature of finish rolling of 850-880° C. and coiled at a temperature of 500-550° C. to obtain a hot-rolled steel sheet having a thickness of 3˜4 mm. Then, the hot-rolled steel sheet is pickled to remove scales and thereafter cold rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm. Next, the cold-rolled steel sheet is subjected to such a continuous annealing that it is heated to a soaking temperature of 750-780° C. and held at this temperature for 40-50 seconds and then cooled at a rate of 20-30° C./second from the soaking temperature to a cooling stop temperature of 350-400° C. and held at the cooling stop temperature range for 100-120 seconds, and then the steel sheet surface is pickled under conditions shown in Table 2, washed with water, dried and subjected to a temper rolling at a stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-25 shown in Table 2.

A test specimen is sampled from each of the above cold-rolled steel sheets and observed at 5 fields of the steel sheet surface with a scanning type electron microscope of ultra-low acceleration voltage (ULV-SEM; made by SEISS; ULTRA 55) at an acceleration voltage of 2 kV, an operating distance of 3.0 mm and a magnification of 1000 times. And analyzed with an energy dispersion X-ray spectrometer (EDX; made by Thermo Fisher; NSS 312E) to obtain a reflection electron image. The reflection electron image is binarized with an image analyzing software (Image J) with respect to gray value (Y-point) corresponding to intersect point (X-point) and threshold value defined in histograms of the aforementioned standard samples Nos. a and b to measure an area ratio of a black portion. The values measured at 5 fields are averaged as a surface covering ratio of iron-based oxide.

Also, a test specimen is sampled from each of the above cold-rolled steel sheets and subjected to a phosphate treatment and a coating treatment under the following conditions and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test and composite cycle corrosion test to evaluate a corrosion resistance after coating. Further, a distribution of O, Si, Mn and Fe in depth direction on the surface of the test specimen sampled from each cold-rolled steel sheet is measured with GDS.

(1) Phosphate Treating Conditions

The test specimen sampled from each cold-rolled steel sheet is subjected to a phosphate treatment with a degreasing agent: FC-E2011, a surface regulator: PL-X and a phosphate treating agent: PALBON PB-L3065, which are made by Nihon

Parkerizing Co., Ltd., so as to provide a phosphate coating adhered amount of 1.7-3.0 g/m² under two conditions of the following standard condition and comparative condition of lowering the phosphate treating temperature to a low temperature.

<Standard Condition>

Degreasing step: treating temperature 40° C., treating time 120seconds Spray degreasing, surface regulating step: pH 9.5, Treating temperature room temperature, treating time 20 seconds Phosphate treating step: temperature of phosphate treating solution 35° C., treating time 120 seconds

<Low Temperature Condition>

Condition of lowering the temperature of the phosphate treating solution in the above standard condition to 33° C.

(2) Corrosion Test

The surface of the test specimen subjected to the phosphate treatment is electrodeposited with an electrodeposition paint : V-50 made by Nippon Paint Co., Ltd. so as to have a coating thickness of 25 μm and then subjected to the following three corrosion tests.

<Hot Salt Water Immersion Test>

The test specimen (n=1) subjected to the phosphate treatment and electrodeposition is provided on its surface with a crosscut flaw of 45 mm in length by means of a cutter, and thereafter immersed in a solution of 5 mass % NaCl (60° C.) for 240 hours, washed with water, and dried. After an adhesive tape is attached to a cut flaw portion, a test of peeling off the tape is carried out to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 5.0 mm, the corrosion resistance can be evaluated to be good in the hot slat water immersion test.

<Salt Water Spray Test (SST)>

The test specimen (n=1) subjected to the phosphate treatment and electrodeposition is provided on its surface with a crosscut flaw of 45 mm in length by means of a cutter, and thereafter subjected to a salt water spray test with an aqueous solution of 5 mass % NaCl for 1000 hours according to a neutral salt water spray test defined in JIS Z2371:2000,and then a tape peeling test on a crosscut flaw portion is conducted to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 4.0 mm, the corrosion resistance can be evaluated to be good in the salt water spray test.

<Composite Cycle Corrosion Test (CCT)>

The test specimen (n=1) subjected to the phosphate treatment and electrodeposition is provided on its surface with a crosscut flaw of 45 mm in length by means of a cutter, and thereafter subjected to a corrosion test that one cycle of salt water spraying (aqueous solution of 5 mass % NaCl: 35° C., relative humidity: 98%) for 2 hours→drying (60° C., relative humidity: 30%) for 2 hours→wetting (50° C., relative humidity: 95%) for 2 hours is repeated 90 cycles, washed with water and dried, and then a tape peeling test on a cut flaw portion is conducted to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 6.0 mm, the corrosion resistance can be evaluated to be good in the composite cycle corrosion test.

The test results are also shown in Table 2. As seen from these results, the steel sheets of Invention Examples subjected to the pickling under the conditions adequate for the invention after the continuous annealing are small in the maximum peeled full width on all of the hot salt water immersion test, salt water spray test and composite cycle corrosion test and show the good corrosion resistance after coating. On the contrary, all of the steel sheets of Comparative Examples not satisfying the pickling conditions of the invention for removing the Si-containing oxide layer on the surface of the steel sheet or having the surface covering ratio of the iron-based oxide of more than 85% are confirmed to be poor in the corrosion resistance after coating. Moreover, as the distribution in depth direction of O, Si, Mn and Fe on the surface of each steel sheet in Table 2 is measured with GDS, it has been confirmed that in the steel sheets pickled under the conditions adequate for the invention, peaks of Si and O do not appear and the Si-containing oxide layer is removed sufficiently. As a reference, FIG. 5 shows the profile in depth direction of O, Si, Mn and Fe as surface-analyzed with GDS with respect to the test specimens of Comparative Example No. 1 and Invention Example No. 18 in Table 2.

TABLE 2 Full width peeled after corrosion test (mm) Temperature of Pickling conditions phosphate treating solution Concentration Surface properties 35° C. 33° C. Concentration of hydrochloric Surface covering Hot salt Salt Composite of nitric acid acid Temperature Treating ratio of water water cycle C_(HNO3) C_(HCl) Ratio R of acid solution time iron-based oxide immersion spray corrosion No. (g/l) (g/l) C_(HCl)/C_(HNO3) (° C.) (seconds) (%) test test test Remarks 1  20 2.0 0.10 40 10 92.3 7.2 6.5 7.3 7.8 Comparative example 2 100 5.0 0.05 40 10 91.5 6.9 6.1 7.3 7.6 Comparative example 3 100 1.0 0.01 40 10 86.1 5.3 4.4 6.2 6.3 Comparative example 4 100 25.0 0.25 40 10 88.3 5.9 5.3 6.5 6.9 Comparative example 5 110 0.5  0.005 40 10 85.5 5.3 4.3 6.2 6.3 Comparative example 6 110 1.1 0.01 40 10 70.8 4.6 3.7 5.7 5.8 Invention example 7 110 11.1 0.10 20 10 57.8 4.4 3.5 5.5 5.7 Invention example 8 110 11.1 0.10 40 3 65.0 4.6 3.5 5.6 5.7 Invention example 9 110 11.1 0.10 40 10 59.4 4.6 3.5 5.6 5.6 Invention example 10 110 11.1 0.10 40 30 56.3 4.5 3.3 5.4 5.6 Invention example 11 110 11.1 0.10 70 10 71.7 4.6 3.6 5.8 5.9 Invention example 12 110 22.2 0.20 40 10 52.9 4.3 3.4 5.4 5.5 Invention example 13 110 27.5 0.25 40 10 45.6 4.3 3.2 5.4 5.5 Invention example 14 150 1.5 0.01 40 3 72.7 4.8 3.6 5.8 5.8 Invention example 15 150 1.5 0.01 40 10 71.7 4.7 3.7 5.7 5.7 Invention example 16 150 1.5 0.01 40 30 67.5 4.7 3.7 5.7 5.7 Invention example 17 150 15.0 0.10 20 10 59.4 4.5 3.6 5.6 5.6 Invention example 18 150 15.0 0.10 40 10 72.6 4.5 3.5 5.6 5.6 Invention example 19 150 15.0 0.10 70 10 78.9 4.8 3.7 5.9 5.8 Invention example 20 150 30.0 0.20 40 10 56.3 4.4 3.3 5.5 5.6 Invention example 21 150 37.5 0.25 40 10 52.9 4.4 3.3 5.6 5.4 Invention example 22 200 2.0 0.01 40 10 78.4 4.7 3.7 5.8 5.9 Invention example 23 200 50.0 0.25 40 10 51.2 4.3 3.3 5.5 5.5 Invention example 24 300 30.0 0.10 40 10 85.5 5.2 4.2 6.1 6.3 Comparative example 25 300 75.0 0.25 40 10 85.1 5.1 4.3 6.2 6.4 Comparative example

Example 2

Each of steels A-X having a chemical composition shown in Table 3 is prepared according to common refining process such as melting in a converter, degassing treatment and the like and continuously cast into a steel slab. The steel slab is hot rolled under hot rolling conditions shown in Table 4 to obtain a hot-rolled steel sheet having a thickness of 3-4 mm, which is pickled to remove scales on the surface of the steel sheet and thereafter cold rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm. Next, the cold-rolled steel sheet is continuously annealed under the conditions shown in Table 4, pickled under conditions shown in Table 5, washed with water, dried and subjected to a temper rolling at a stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-30.

TABLE 3 Steel Chemical composition (mass %) symbol C Si Mn P S Al Si/Mn Nb, Ti, V, Mo, Cr, B, N Ni, Cu, Ca, REM Remarks A 0.11 1.25 1.55 0.018 0.001 0.032 0.81 — — Invention steel B 0.15 1.30 1.80 0.019 0.002 0.033 0.72 — — Invention steel C 0.15 1.20 1.95 0.017 0.001 0.033 0.62 — — Invention steel D 0.09 1.45 1.40 0.017 0.002 0.028 1.04 — — Invention steel E 0.18 1.11 1.36 0.018 0.001 0.032 0.82 — — Invention steel F 0.16 1.41 1.23 0.017 0.001 0.041 1.15 — — Invention steel G 0.14 1.65 1.33 0.018 0.002 0.035 1.24 — — Invention steel H 0.12 1.45 2.10 0.017 0.001 0.042 0.69 — — Invention steel I 0.17 0.90 1.40 0.017 0.002 0.044 0.64 — — Invention steel J 0.13 1.20 1.89 0.018 0.001 0.041 0.63 — — Invention steel K 0.15 1.20 1.85 0.017 0.001 0.034 0.65 — — Invention steel L 0.03 1.25 3.25 0.018 0.001 0.005 0.38 — — Invention steel M 0.22 3.30 1.15 0.018 0.001 0.027 2.87 — — Comparative steel N 0.06 1.28 2.12 0.025 0.003 0.040 0.60 Nb: 0.1, Ti: 0.2 Cu: 0.15 Invention steel O 0.18 1.21 1.97 0.015 0.002 0.035 0.61 V: 0.1, Mo: 0.2 Ni: 0.13 Invention steel P 0.18 1.56 2.58 0.010 0.002 0.030 0.60 Cr: 0.2, B: 0.005 Ca: 0.003 Invention steel Q 0.13 1.32 1.32 0.030 0.001 0.040 1.00 N: 0.007 REM: 0.002 Invention steel R 0.07 1.26 2.10 0.025 0.002 0.040 0.60 Nb: 0.1 — Invention steel S 0.06 1.28 2.12 0.025 0.003 0.040 0.60 Nb: 0.1, Ti: 0.2 — Invention steel T 0.17 1.23 1.99 0.015 0.002 0.050 0.62 — Ni: 0.13 Invention steel U 0.18 1.22 1.97 0.015 0.003 0.040 0.62 — Ni: 0.13, Ca: 0.003 Invention steel V 0.18 1.21 1.98 0.015 0.002 0.035 0.61 V: 0.1 Ni: 0.13 Invention steel W 0.18 1.56 2.58 0.010 0.002 0.030 0.60 Mo: 0.1, Cr: 0.2, B: 0.005 Ca: 0.003 Invention steel X 0.13 1.32 1.32 0.030 0.001 0.040 1.00 Nb: 0.1, N: 0.007 Cu: 0.2, REM: 0.002 Invention steel

TABLE 4 Continuous annealing conditions Hot rolling conditions Cooling Heating Finish Coiling Cold Heating stop tem- tem- Cooling tem- rolling tem- Holding Cooling tem- Holding Cooling Strength Steel perature perature rate perature reduction perature time rate perature time rate TS No. symbol (° C.) (° C.) (° C./s) (° C.) (%) (° C.) (Seconds) (° C./s) (° C.) (Seconds) (° C./s) (MPa) Remarks 1 A 1150 850 25 620 60 780 45 20 350 100 40 625 Invention example 2 B 1150 820 31 400 60 780 40 20 400 100 50 821 Invention example 3 C 1140 850 26 600 60 760 50 20 350 100 45 814 Invention example 4 D 1150 840 33 530 60 730 40 20 350 110 40 623 Invention example 5 E 1150 850 30 580 55 750 35 20 400 110 50 836 Invention example 6 F 1150 850 25 620 60 750 50 20 350 120 50 634 Invention example 7 G 1150 850 33 550 60 750 30 20 400 100 50 632 Com- parative example 8 G 1150 850 33 550 60 750 30 20 400 100 50 635 Com- parative example 9 G 1150 850 33 550 60 750 30 20 400 100 50 631 Invention example 10 G 1150 850 33 550 60 750 30 20 400 100 50 633 Com- parative example 11 H 1130 820 28 570 60 780 50 15 370 150 50 840 Invention example 12 I 1150 840 34 530 55 780 50 15 350 120 55 812 Invention example 13 J 1140 850 28 600 60 770 60 20 300 100 45 836 Invention example 14 K 1150 850 25 620 60 780 45 20 350 100 40 650 Invention example 15 L 1100 850 33 550 60 750 50 20 450 150 50 960 Com- parative example 16 L 1100 850 33 550 60 750 50 20 450 150 50 959 Com- parative example 17 L 1100 850 33 550 60 750 50 20 450 150 50 963 Invention example 18 L 1100 850 33 550 60 750 50 20 450 150 50 962 Com- parative example 19 M 1120 830 31 550 55 720 50 15 410 190 50 1124 Com- parative example 20 N 1120 830 33 550 60 750 30 20 400 100 50 613 Invention example 21 O 1150 850 32 560 60 750 35 20 350 100 50 776 Invention example 22 P 1130 840 33 550 55 780 30 20 400 110 50 1152 Invention example 23 Q 1140 850 33 580 60 750 40 20 400 120 45 586 Invention example 24 R 1120 830 33 550 60 750 30 20 400 100 50 611 Invention example 25 S 1120 830 32 550 60 750 35 20 410 100 50 621 Invention example 26 T 1150 850 32 560 60 750 35 20 350 100 50 773 Invention example 27 U 1140 850 33 550 60 750 35 20 400 100 50 785 Invention example 28 V 1150 850 32 550 60 750 40 20 380 100 50 770 Invention example 29 W 1130 840 33 550 55 780 30 20 400 110 50 1156 Invention example 30 X 1140 850 33 580 60 750 40 20 400 120 45 585 Invention example

TABLE 5 Full width peeled Surface after corrosion test(mm) properties Temperature of phosphate Pickling conditions Surface treating solution Concentration covering 35° C. 33° C. Concentration of hydrochloric Temperature ratio of Hot salt Salt Composite of nitric acid acid of Treating iron-based water water cycle Steel C_(HNO3) C_(HCl) Ratio R acid solution time oxide immersion spray corrosion No. symbol (g/l) (g/l) C_(HCl)/C_(HNO3) (° C.) (Seconds) (%) test test test Remarks 1 A 150 15 0.10 40 10 54.6 4.3 3.8 5.7 5.8 Invention example 2 B 150 15 0.10 40 10 57.8 4.2 3.7 5.6 5.8 Invention example 3 C 150 15 0.10 40 10 52.9 4.2 3.5 5.3 5.5 Invention example 4 D 150 15 0.10 40 10 54.6 4.3 3.6 5.5 5.6 Invention example 5 E 150 15 0.10 40 10 49.3 4.2 3.5 5.6 5.7 Invention example 6 F 150 15 0.10 40 10 56.3 4.4 3.7 5.5 5.7 Invention example 7 G  50 5 0.10 40 10 92.5 7.3 6.6 7.5 7.8 Comparative example 8 G 150 1  0.007 40 10 85.6 5.5 4.3 6.1 6.6 Comparative example 9 G 150 15 0.10 40 10 57.8 4.4 3.6 5.6 5.7 Invention example 10 G 250 25 0.10 40 10 85.3 5.1 4.2 6.2 6.3 Comparative example 11 H 150 15 0.10 40 10 56.3 4.1 3.9 5.5 5.6 Comparative example 12 I 150 15 0.10 40 10 66.3 4.4 3.8 5.3 5.5 Invention example 13 J 150 15 0.10 40 10 62.3 4.3 3.5 5.1 5.5 Invention example 14 K 150 15 0.10 40 10 57.4 4.1 3.2 5.6 5.1 Invention example 15 L  50 5 0.10 40 10 92.3 7.2 6.5 7.3 7.8 Comparative example 16 L 150 1  0.007 40 10 85.5 5.3 4.3 6.2 6.3 Comparative example 17 L 150 15 0.10 40 10 59.4 4.5 3.6 5.8 5.6 Invention example 18 L 250 25 0.10 40 10 85.5 5.2 4.2 6.1 6.3 Comparative example 19 M 150 15 0.10 40 10 87.0 5.3 4.2 6.2 6.5 Comparative example 20 N 150 15 0.10 40 10 53.9 4.3 3.5 5.4 5.5 Invention example 21 O 150 15 0.10 40 10 55.5 4.1 3.6 5.7 5.4 Invention example 22 P 150 15 0.10 40 10 54.2 4.4 3.5 5.6 5.6 Invention example 23 Q 150 15 0.10 40 10 54.8 4.3 3.6 5.5 5.5 Invention example 24 R 150 15 0.10 40 10 53.6 4.2 3.3 5.6 5.4 Invention example 25 S 150 15 0.10 40 10 55.5 4.1 3.5 5.6 5.6 Invention example 26 T 150 15 0.10 40 10 54.5 4.3 3.4 5.5 5.7 Invention example 27 U 150 15 0.10 40 10 54.7 4.2 3.5 5.4 5.5 Invention example 28 V 150 15 0.10 40 10 54.5 4.2 3.5 5.5 5.7 Invention example 29 W 150 15 0.10 40 10 54.7 4.2 3.6 5.6 5.5 Invention example 30 X 150 15 0.10 40 10 54.3 4.3 3.5 5.6 5.5 Invention example

A test specimen is sampled from each of the cold-rolled steel sheets and subjected to the following tensile test and test for the corrosion resistance after coating after the surface covering ratio of iron-based oxide on the steel sheet surface after the pickling is measured in the same manner as in Example 1. Also, the distribution in depth direction of O, Si, Mn and Fe on the surface of the test specimen sampled from each of the cold-rolled steel sheets is measured with GDS.

(1) Mechanical Properties

A tensile test specimen of JIS No. 5 (n=1) sampled in a direction (C-direction) parallel to the rolling direction according to JIS Z2201:1998 is subjected to a tensile test according to JIS Z2241:1998 to measure tensile strength TS.

(2) Corrosion Resistance After Coating

A test specimen is prepared by subjecting the test specimen sampled from each of the cold-rolled steel sheet to phosphate treatment and electrodeposition under the same conditions as in Example 1 and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test (SST) and composite cycle corrosion test (CCT) likewise Example 1 to evaluate the corrosion resistance after coating.

The results of the above tests are shown in Tables 4 and 5. As seen from these results, the high-strength cold-rolled steel sheets of Invention Examples containing Si of not less than 0.5 mass % and pickled under the conditions adequate for the invention are excellent in the corrosion resistance after coating but also have a tensile strength TS of not less than 590 MPa. Moreover, as the distribution in depth direction of O, Si, Mn and Fe is measured with GDS, it has been confirmed that in all of the steel sheets pickled under the conditions adequate for the invention, peaks of Si and O do not appear and the Si-containing oxide layer is removed sufficiently.

Example 3

A steel comprising C: 0.125 mass %, Si: 1.5 mass %, Mn: 2.6 mass %, P: 0.019 mass %, S: 0.008 mass %, Al: 0.040 mass % and the remainder being Fe and inevitable impurities is melted and continuously cast into a steel material (slab). The slab is reheated to a temperature of 1150-1170° C., hot rolled at a terminating temperature of finish rolling of 850-880° C. and coiled at a temperature of 500-550° C. to obtain a hot-rolled steel sheet having a thickness of 3-4 mm. The hot-rolled steel sheet is pickled to remove scales and thereafter cold rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm. Next, the cold-rolled steel sheet is subjected to such a continuous annealing that it is heated to a soaking temperature of 750-780° C. and held at this temperature for 40-50 seconds and then cooled at a rate of 20-30° C./second from the soaking temperature to a cooling stop temperature of 350-400° C. and held at the cooling stop temperature range for 100-120 seconds, and then the steel sheet is pickled under conditions shown in Table 6, washed with water, dried and subjected to a temper rolling at a stretching ratio of 0.7% to obtain cold-rolled steel sheets Nos. 1-12 shown in Table 6.

A test specimen is sampled from each of the above cold-rolled steel sheets to measure a surface covering ratio and maximum thickness of iron-based oxide generated on the surface of the steel sheet by pickling through the aforementioned methods.

Also, the test specimen is sampled from each of the above cold-rolled steel sheets and subjected to phosphate treatment and coating treatment under the following conditions and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test and composite cycle corrosion test to evaluate the corrosion resistance after coating.

Also, the test specimen is sampled from each of the above cold-rolled steel sheets and subjected to phosphate treatment and coating treatment under the following conditions and then subjected to three corrosion tests of hot salt water immersion test, salt water spray test and composite cycle corrosion test to evaluate the corrosion resistance after coating. Further, the distribution in depth direction of O, Si, Mn and Fe on the surface of the test specimen sampled from each of the cold-rolled steel sheets is measured with GDS.

(1) Phosphate Treating Conditions

The test specimen sampled from each cold-rolled steel sheet is subjected to a phosphate treatment with a degreasing agent: FC-E2011, a surface regulator: PL-X and a phosphate treating agent: PALBOND PB-L3065, which are made by Nihon Parkerizing Co., Ltd., so as to provide a phosphate coating adhered amount of 1.7-3.0 g/m2 under two conditions of the following standard condition and comparative condition of lowering the phosphate treating temperature to a low temperature.

<Standard Condition>

Degreasing step: treating temperature 40° C., treating time 120 seconds Spray degreasing, surface regulating step: pH 9.5, Treating temperature room temperature, treating time 20 seconds Phosphate treating step: temperature of phosphate treating solution 35° C., treating time 120 seconds <Low temperature condition> Condition of lowering the temperature of the phosphate treating solution in the above standard condition to 33° C.

(2) Corrosion Test

The surface of the test specimen subjected to the phosphate treatment is electrodeposited with an electrodeposition paint : V-50 made by Nippon Paint Co., Ltd. so as to have a coating thickness of 25 μm and then subjected to the following three corrosion tests with more strict condition than the one with Example 1.

<Hot Salt Water Immersion Test>

The test specimen (n=1) subjected to the phosphate treatment and electrodeposition is provided on its surface with a crosscut flaw of 45 mm in length by means of a cutter, and thereafter immersed in a solution of 5 mass % NaCl (60° C.) for 360 hours, washed with water, and dried. After an adhesive tape is attached to a cut flaw portion, a test of peeling off the tape is carried out to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 5.0 mm, the corrosion resistance can be evaluated to be good in the hot slat water immersion test.

<Salt Water Spray Test (SST)>

The test specimen (n=1) subjected to the phosphate treatment and electrodeposition is provided on its surface with a crosscut flaw of 45 mm in length by means of a cutter, and thereafter subjected to a salt water spray test with an aqueous solution of 5 mass % NaCl for 1200 hours according to a neutral salt water spray test defined in JIS Z2371:2000,and then a tape peeling test on a crosscut flaw portion is conducted to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 4.0 mm, the corrosion resistance can be evaluated to be good in the salt water spray test.

<Composite Cycle Corrosion Test (CCT)>

The test specimen (n=1) subjected to the phosphate treatment and electrodeposition is provided on its surface with a crosscut flaw of 45 mm in length by means of a cutter, and thereafter subjected to a corrosion test that one cycle of salt water spraying (aqueous solution of 5 mass % NaCl: 35° C., relative humidity: 98%) for 2 hours→drying (60° C., relative humidity: 30%) for 2 hours→wetting (50° C., relative humidity: 95%) for 2 hours is repeated 120 cycles, washed with water and dried, and then a tape peeling test on a cut flaw portion is conducted to measure a maximum peeled full width combining either side of the cut flaw portion. When the maximum peeled full width is not more than 6.0 mm, the corrosion resistance can be evaluated to be good in the composite cycle corrosion test.

The test results are also shown in Table 6. As seen from these results, the steel sheets of Invention Examples, wherein the surface of the steel sheet after annealing is subjected to the pickling under the conditions that the surface covering ratio of the iron-based oxide on the surface of the steel sheet after pickling is not more than 85% and the maximum thickness of the iron-based oxide is not more than 200 nm, are small in the maximum peeled full width on all of the hot salt water immersion test, salt water spray test and composite cycle corrosion test and show the very good corrosion resistance after coating. Moreover, as the distribution in depth direction of O, Si, Mn and Fe is measured with GDS, it has been confirmed that in the steel sheets pickled under the conditions adequate for the invention, peaks of Si and O do not appear and the Si-containing oxide layer is removed sufficiently.

TABLE 6 Full width peeled after corrosion test (mm) Surface properties Temperature of phosphate Pickling conditions Surface Maximum treating solution Concentration covering thickness 35° C. 33° C. Concentration of hydrochloric Temperature ratio of of Hot salt Salt Composite of nitric acid acid of Treating iron-based iron-based water water cycle C_(HNO3) C_(HCl) Ratio R acid solution time Oxide oxide immersion spray corrosion No. (g/l) (g/l) C_(HCl)/C_(HNO3) (° C.) (Seconds) (%) (nm) test test test Remarks 1 110 1.1 0.01 40 10 70.8 212 6.6 5.9 8.2 8.4 Invention example 2 110 3.3 0.03 40 10 67.3 196 5.0 3.9 5.9 6.0 Invention example 3 110 11.1 0.10 40 10 59.4 179 4.8 3.7 5.8 5.7 Invention example 4 110 22.2 0.20 40 10 52.9 167 4.6 3.5 5.7 5.7 Invention example 5 110 27.5 0.25 40 10 45.6 161 4.5 3.5 5.5 5.6 Invention example 6 140 1.4 0.01 40 10 71.4 216 6.7 5.8 8.2 8.2 Invention example 7 140 4.2 0.03 40 10 68.1 198 4.9 4.0 6.0 5.8 Invention example 8 140 14.0 0.10 40 10 64.8 184 4.9 3.9 5.8 6.0 Invention example 9 140 28.0 0.20 40 10 55.1 172 4.6 3.7 5.7 5.8 Invention example 10 140 35.0 0.25 40 10 51.7 167 4.6 3.6 5.7 5.6 Invention example 11 150 15.0 0.10 40 10 72.6 214 6.3 5.5 7.9 8.3 Invention example 12 150 37.5 0.25 40 10 52.9 203 6.1 5.3 7.6 8.1 Invention example

INDUSTRIAL APPLICABILITY

The cold-rolled steel sheets produced according to the invention not only are excellent in the corrosion resistance after coating but also have a high strength and a good workability, so that they can be preferably used as not only a starting material used in members of the automotive vehicle body but also a starting material for applications requiring the same properties such as household electrical goods, building members and so on. 

1. A method of producing a cold-rolled steel sheet having Si with a range of 0.5-3.0 mass %, characterized in that a continuously annealed steel sheet after cold rolling is pickled with a mixture of nitric acid and hydrochloric acid having a nitric acid concentration of more than 100 g/L but not more than 200 g/L and a ratio R (HCl/HNO₃) of hydrochloric acid concentration to nitric acid concentration of 0.01-0.25.
 2. A method of producing a cold-rolled steel sheet according to claim 1, wherein the mixture of nitric acid and hydrochloric acid has a nitric acid concentration of more than 110 g/L but not more than 140 g/L and a ratio R (HCl/HNO₃) of hydrochloric acid concentration to nitric acid concentration of 0.03-0.25.
 3. A method of producing a cold-rolled steel sheet according to claim 1, wherein the pickling is carried out at a temperature of a pickling solution of 20-70° C. for 3-30 seconds.
 4. A method of producing a cold-rolled steel sheet according to claim 1, wherein the steel sheet has a chemical composition comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities.
 5. A method of producing a cold-rolled steel sheet according to claim 1, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.
 6. A method of producing a cold-rolled steel sheet according to claim 1, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
 7. A cold-rolled steel sheet produced by a method as claimed in claim 1, characterized in that a Si-containing oxide layer is removed from the surface of the steel sheet by pickling after continuous annealing and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85%.
 8. A cold-rolled steel sheet according to claim 7, wherein a maximum thickness of the iron-based oxide existing on the surface of the steel sheet is not more than 200 nm.
 9. A member for automobile characterized by using a cold-rolled steel sheet as claimed in claim
 7. 10. A method of producing a cold-rolled steel sheet according to claim 2, wherein the pickling is carried out at a temperature of a pickling solution of 20-70° C. for 3-30 seconds.
 11. A method of producing a cold-rolled steel sheet according to claim 2, wherein the steel sheet has a chemical composition comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities.
 12. A method of producing a cold-rolled steel sheet according to claim 3, wherein the steel sheet has a chemical composition comprising, in addition to Si, C: 0.01-0.30 mass %, Mn: 1.0-7.5 mass %, P: not more than 0.05 mass %, S: not more than 0.01 mass % and Al: not more than 0.06 mass % and the remainder being Fe and inevitable impurities.
 13. A method of producing a cold-rolled steel sheet according to claim 2, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.
 14. A method of producing a cold-rolled steel sheet according to claim 3, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.
 15. A method of producing a cold-rolled steel sheet according to claim 4, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Nb: not more than 0.3 mass %, Ti: not more than 0.3 mass %, V: not more than 0.3 mass %, Mo: not more than 0.3 mass %, Cr: not more than 0.5 mass %, B: not more than 0.006 mass % and N: not more than 0.008 mass %.
 16. A method of producing a cold-rolled steel sheet according to claim 2, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
 17. A method of producing a cold-rolled steel sheet according to claim 3, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
 18. A method of producing a cold-rolled steel sheet according to claim 4, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
 19. A method of producing a cold-rolled steel sheet according to claim 5, wherein the steel sheet comprises, in addition to the above chemical composition, one or more selected from the group consisting of Ni: not more than 2.0 mass %, Cu: not more than 2.0 mass %, Ca: not more than 0.1 mass % and REM: not more than 0.1 mass %.
 20. A cold-rolled steel sheet produced by a method as claimed in claim 2, characterized in that a Si-containing oxide layer is removed from the surface of the steel sheet by pickling after continuous annealing and a ratio of covering the surface of the steel sheet with an iron-based oxide formed by the pickling is not more than 85%. 